An EEPROM or electrically erasable and programmable read only memory is a non-volatile storage unit used for example in computers or other devices. The EEPROM storage unit comprises a plurality of memory cells that can be programmed and erased electrically multiple times.
The EEPROM memory cell may be formed as a floating gate memory cell or as a so-called nitride programmable read only memory cell (NROM memory cell). Depending on the form of the memory cell, one or more bits can be stored in the memory cell.
Each memory cell comprises a transistor body having two terminals serving as source or drain. A gate electrode is arranged on the transistor body and a dielectric layer is located between the gate electrode and the transistor body.
The memory cells are usually arranged in an array having rows and columns. A multitude of memory cells is arranged in one of the rows and a multitude of memory cells is arranged in one of the columns.
The array may be organized in sectors. The gate electrodes of the memory cells located in a same row of the sector are coupled to a same wordline of a plurality of wordlines. The terminals of the memory cells located in a same column are coupled to a first bitline and to a second bitline, respectively. The first bitline and the second bitline are located on either side of these memory cells.
Access to each of these memory cells may be performed by applying access potentials, including reading potentials, programming potentials and erasing potentials, to the wordline and to the first and the second bitlines coupled to the memory cell to be accessed.
Erasing of the memory cells may be performed in parallel. The erasing potentials are applied to all wordlines and bitlines coupled to the memory cells of the erasing sector. Alternatively, the memory cells of a block may be erased in portions of the block. It also may be possible to read and program the memory cells located in the same row in parallel. The reading and programming potentials are applied to the wordline and selected bitlines that are coupled to the memory cells to be programmed or to be read. The reading potentials or the programming potentials are applied to the bitlines, located on either side of these memory cells, and to selected wordlines coupled to the memory cells to be read or to be programmed.
A special form of the EEPROM is a so-called flash memory that can be erased in parallel or block wise. Storing information in the memory cells of the flash memory is performed by first erasing all memory cells of the erasing sector. Then selected memory cells are programmed in order to store a bit pattern representing the information to be stored in the memory cells.
Programming and erasing is performed by applying the programming potentials and the erasing potentials, respectively, to the memory cells. The potentials are usually applied in pulses. Then a so-called full verifying step is performed in order to verify whether each of the memory cells to be programmed is programmed or whether each of the memory cells to be erased is erased. The actual erasing or programming step followed by the full verifying step is repeated until all memory cells are successfully verified, which means all memory cells to be erased are verified as erased or all memory cells to be programmed are verified as programmed.
Usually all memory cells of the erasing sector, including the memory cells that already have been erased before performing the first erasing step, are verified. Full verifying whether programming has been successfully performed is done in a similar way. In this case the group of memory cells to be programmed is verified.
The above-mentioned process for verification is very time consuming because each memory cell is verified. Furthermore, the process is energy consuming because verifying potentials are applied to each memory cell for verifying.